The invention relates to a valve with a packed seal. Davis et al., U.S. Pat. No. 4,886,241 describes such a valve with a packed seal, which is arranged between the housing and the piston connected with the housing and axially displaceable or rotatable within the housing. In valves for media with a high melting point, which operate near the melting point, there is a risk of damaging the packed seal due to xe2x80x9cfreezingxe2x80x9d of the medium as a result of the adhesive properties of the medium in temperature ranges below the melting point. In principle, these problems can be counteracted by heating the housing, but the measures required for this have thus far been relatively complex and costly.
Accordingly, it is an object of the invention to provide a reliable packed seal valve structure with low design complexity.
Another object of the invention is to provide a packed seal valve in which the packed seal operates reliably within the temperature range at which the medium is liquid.
A further object of the invention is to provide a simply constructed valve in which, despite its simple construction, the packed seal material is protected from the freezing point and the temperature of the packed seal reliably remains in the liquid range above the melting point of the medium flowing through the valve.
These and other objects are achieved in accordance with the claimed invention by providing a valve comprising a piston connected with a valve body and supported in a housing, and a packed seal arranged between the piston and the housing, wherein the housing is provided in the area of the packed seal with a heating device having an axial length is greater than that of the packed seal, and wherein the heating device protrudes axially beyond at least one axial end of the packed seal by a predetermined length.
The proposed valve with a packed seal is distinguished by its functionally reliable construction and ensures with high reliability that the temperature of the packed seal is always within the liquid range above the melting point of the medium. In the area of the packed seal, the housing comprises a heating device having an axial length which is greater by a predefined amount than the length of the packed seal. At least one end, and desirably both ends, of the heating device will protrude beyond the packed seal in a predefined area. Advantageously, the heating device will protrude beyond the end of the packed seal by a length of 30 to 60 mm.
The heating device advantageously comprises a heating jacket surrounding the exterior of the housing. A gap for a heating medium, particularly a heatable heat transfer fluid, is thus provided between the housing and the heating jacket. Alternatively, within the scope of the invention, the heating device can comprise tubes through which a heating medium flows, or an electrical heating coil or the like, which are arranged especially on the exterior surface of the housing in the area of the packed seal.
Furthermore, in the area of the packed seal and a support bushing adjacent to the packed seal, the piston is advantageously designed to have a constant diameter, where no recesses, annular collars or the like are present. Particularly advantageously, the support bushing, which is provided for guiding the piston, is designed with very tight tolerances relative to the housing on the one hand and the piston on the other hand. This is particularly important for sensitive media, which are subject to change as a function of temperature and/or retention time, especially since the selected tolerance affects the quality of sensitive media.
In view of the tolerances necessary for the required quality of the medium, the material of the support bushing according to the invention has at least approximately the same thermal expansion characteristics as the material of the housing and/or the piston. In addition, the piston guide is advantageously substantially harder than the housing and/or the piston material in order to prevent xe2x80x9ccorrosionxe2x80x9d or xe2x80x9cwearxe2x80x9d in the guide for the piston. Within the scope of the invention, the tolerances are preferably in the range of 0.01 mm to 0.2 mm, particularly preferably about 0.05 mm.
Further developments and special embodiments of the invention are set forth hereinafter.